This turned out not to be ideal, though, on further thought - it means bringing the 'published/unpublished' field into 'normal' form view, and because I have the start date within a section, if anyone did want to create an unpublished regular event, they'd (counterintuitively) have to find that setting in the 'special events' group. It also doesn't respect the date format setting.

However there is what looks a better way. I believe that if I put this into my template: